Method of using a heater with a fiber optic string in a wellbore

ABSTRACT

A heater cable is deployed in a well bore to elevate the temperature of the wellbore above the temperature of the surrounding fluid and the formation. One or more fiber optic strings are included in or carried by the heater cable which is placed along a desired length of the wellbore. At least one fiber optic string measures temperature of the heater cable at a plurality of spaced apart locations. Another string is utilized to determine the temperature of the wellbore. The heater cable is heated above the temperature of the well bore. The fluid flowing from the formation to the wellbore lowers the temperature of the cable at the inflow locations. The fiber optic string provides measurements of the temperature along the heater cable. The fluid flow is determined from the temperature profile of the heater cable provided by the fiber optic sensors.

RELATED APPLICATIONS

This is a divisional of U.S. Ser. No 09/763,543, filed May 22, 2001, nowU.S. Pat. No. 6,497,279, the disclosure of which is incorporated hereinby reference. U.S. Ser. No 09/763,543 is 371 of PCT InternationalApplication PCT/US99/19781, filed Aug. 25, 1999, which claims priorityfrom U.S. Provisional Application Serial No. 60/097,783, filed Aug. 25,1998.

1. FIELD OF THE INVENTION

This invention relates to utilizing fiber optic sensor strings withheater cables for use in oil wells and more particularly for determiningthe flow of formation fluid into the wellbore and to control theoperation of the heater cables for optimum operations.

2. BACKGROUND OF THE ART

Heater cables are often used in wellbores to increase the temperature ofthe fluid in the wellbore to prevent the formation of paraffins and toprevent the oil from flocculating. Such phenomena cause at least some ofthe oil to become highly viscous, and often plugs the perforations. Suchfluids can clog the electrical submersible pumps. Heater cables are alsoused to heat the formation surrounding the wellbores which contain heavy(highly viscous) oil to reduce the viscosity of such oil.

The heater cable usually is a resistance heating element. High currentsupplied from the surface can heat the cable to a temperaturesubstantially higher than the formation temperature. In ESPapplications, a heater current may be deployed below the ESP. In otherproduction wells, heater cable may be installed along any desiredportion or segment of the wells. It is desirable to determine the fluidflow from various production zones along a wellbore and to monitor andcontrol the temperature of the heater cable so as to heat the wellboreonly as required for optimum recovery and to reduce power consumption.

U.S. Pat. No. 4,435,978 discloses a hot wire anemometer in which heat issupplied at a constant rate to a sensor element with fluid flowing pastthe element. The drop in temperature of the sensor element is used togive a measurement of the fluid flow. This method accurately measuresthe flow under a variety of flow conditions.

U.S. Pat. No. 5,551,287 discloses a wireline device in which a hot filmanemometer deployed on sensor pads measures the temperature of fluidentering the borehole. The fluid flowing past the sensor elementproduces a change in resistance that is used in a bridge circuit to givea measurement of temperature. This temperature measurement, whencombined with a measurement of local ambient temperature, gives anindication of the rate of fluid flow into the borehole. U.S. Pat. No.4,621,929 discloses a fiber optic thermal anemometer using a sensorelement with temperature sensitive optical properties.

The present invention is an apparatus and method for monitoring thefluid flow from a producing well with a plurality of producingintervals. A cable that includes a number of fiber optic thermalanemometer sensors is deployed in the producing well with the sensors inthe vicinity of the perforations in the casing or inlets from whichfluid from the reservoirs enters the production casing. The presentinvention also provides temperature distribution along the heater cablelength which information is utilized to control the operation of theheater cable.

SUMMARY OF THE INVENTION

The present invention provides a heater cable that may be deployed in awellbore to elevate the temperature of the wellbore above thetemperature of the surrounding fluid and the formation. One or morefiber optic strings are included in or are carried by the heater cable.The heater cable carrying the fiber optics is placed along the desiredlength of the wellbore. At least one fiber optic string measurestemperature of the heater cable at a plurality of spaced apartlocations. Another string may be utilized to determine the temperatureof the wellbore. In one aspect of this invention, the heater cable isheated above the temperature of the wellbore. The fluid flowing from theformation to the wellbore lowers the temperature of the cable at theinflow locations. The fiber optic string provides measurements of thetemperature along the heater cable. The fluid flow is determined fromthe temperature profile of the heater cable provided by the fiber opticsensors. In another aspect of this invention, the temperaturedistribution along the heater cable is used to control the operation ofthe heater cable to maintain the elevated temperature within desiredlimits. The heater cable may be selected turned on and turned off toprovide only the desired amount of heat. This may be accomplished byselectively turning on and turning off the heater cable or by increasingand decreasing the electric power supplied as a function of the downholemeasured temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

For detailed understanding of the present invention, references shouldbe made to the following detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, inwhich like elements have been given like numerals, wherein:

FIG. 1A shows a portion of a heater cable carrying fiber optic stringsaccording to the present invention.

FIG. 1B shows the cable of FIG. 1A deployed in a producing borehole thatpenetrates a number of reservoirs.

FIG. 2 shows a heater cable deployed in a wellbore being controlled by acontrol unit as a function of the temperature measurements provided bythe fiber optic sensors in the cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows a portion of a cable 10 according to one embodiment of thepresent invention. It includes a heater cable 20 that carries electricalcurrent and is used as a source of heat by means of uniformly dispersedresistive elements within a portion of the cable (not shown). The cablealso includes a pair of fibers 30 a, 30 b for carrying optical signalsdown the borehole and back up the borehole and for measuring temperatureat spaced locations along the fibers.

FIG. 1B shows the cable 10 deployed in a producing borehole 50 thatpenetrates a number of reservoirs. For illustrative purposes, threeproducing intervals 40 a, 40 b and 40 c are shown. For illustrativepurposes only, each of the three producing intervals is assumed to havea uniform temperature of T₀. Further, each of the three producingintervals has a different rate of flow, denoted by Q₁, Q₂, Q₃, ofreservoir fluid into the producing well 50. A plurality of fiber opticsensors, 60 a, 60 b, 60 c . . . 60 n in the cable 10 make continuousmeasurements of temperature at the respective locations. To determinethe flow rate from the various zones Q1-Q3, a control unit 60 providespower to the heater cable 20, to cause it to heat the wellbore 50 to atemperature T1 that is significantly higher than To. The fluid flowingfrom the zones 40 a-40 e, which causes the temperature of the heatercable 20 to drop at the flow locations. Under these conditions, thegreater the flow rate of fluid Q₁ past a sensor, the greater thetemperature of the sensor will drop from T₁ towards T₀. Measurements oftemperature of the sensor are used as an indication of the flow of theformation fluid into the wellbore 50.

The control unit 60 receives the signals from the fiber optic strings 30a-30 b and can be programmed to calculate the fluid flow from each zone.A log such as shown by the resultant log 70 may be continuouslydisplayed and recorded by the control unit 60. The log 70 shows atemperature profile along the well 50. An example of the affect on thetemperature curve 80 of the flow from zones Q1-Q3 respectively is shownat locations 80 a-80 c.

FIG. 2 shows a heater cable 100 made according to the present invention,deployed or placed in a wellbore 110 having a casing 112. The cable 100includes one or more fiber optic string 120 adapted to measuretemperature at spaced apart locations T1-Tn along a segment or portionof the cable 100 shown by the dotted line. The heater cable 100 isadapted to heat any desired segment of the cable. For convenience, theheater cable herein is assumed to carry heating elements that heat thesegment from T1-Tn. A power unit 130 supplies power to the heatingelement 111. A control unit 140 controls the power unit 130, and anoptical energy and data unit 142.

The heater element 111 is heated to a predetermined temperature toenhance production flow to the surface. The fiber optic stringcontinuously provides the temperature profile along the wellbore viasensor T1-Tn. If the temperature of the cable 100 in the wellbore isoutside a predetermined norm, the control unit 140 adjusts the power tothe cable 100 until the heater cable temperature provided by T1-Tn fallsback in the desired limits. The control unit may be programmed toselectively turn on and turn off the heater cable to optimize the powerconsumption and to enhance the operating life of the heater cable.

The heater cable 100 may be deployed below an electrical submersiblepump (ESP) when used as shown in FIG. 2 and also above the ESP. Thetemperature distribution T1-Tn along the heater cable is also useful inpredicting heater cable 100 failures. It provides indication of hotspots in the heater cable and the efficiency of the cable correspondingto the input power.

Since the current supplied to the heater element 111 is the same, theheat generated by a uniform heater element will be uniform. Thetemperature distribution T1-Tn can thus provide indication of thequality of the heater cable's 110 performance.

While the foregoing disclosure is directed to the preferred embodimentsof the invention, various modifications will be apparent to thoseskilled in the art. It is intended that all variations within the scopeand spirit of the appended claims be embraced by the foregoingdisclosure.

What is claimed is:
 1. A method for use in a wellbore, comprising:providing a heater cable and a distributed temperature sensor; placingthe heater cable and the distributed temperature sensor in the wellbore;elevating the temperature of at least a segment of the surroundingwellbore by heating the heater cable to a temperature above atemperature of said at least a segment; measuring a temperature profilealong said at least a segment of the wellbore by use of the distributedtemperature sensor; and determining a characteristic of at least onecontent of the wellbore by using the temperature profile.
 2. The methodof claim 1, wherein the providing step comprises providing the heatercable and the distributed temperature sensor in one bundle.
 3. Themethod of claim 1, wherein the distributed temperature sensor comprisesat least one fiber optic string.
 4. The method of claim 1, wherein theplacing step comprises placing the heater cable and the distributedtemperature sensor along at least one formation of the wellbore.
 5. Themethod of claim 4, wherein said at least one content of the wellborecomprises fluid from the formation and the characteristic comprises flowof the fluid from the formation into the wellbore.
 6. The method ofclaim 4, wherein the placing step comprises placing the heater cable andthe distributed temperature sensor along a plurality of formations ofthe wellbore simultaneously.
 7. The method of claim 6, wherein said atleast one content of the wellbore comprises fluid from the formation andthe characteristic comprises fluid flow from each of the formations intothe wellbore.
 8. The method of claim 1, further comprising adjusting theheating level of the heater cable as a function of the temperatureprofile.
 9. The method at claim 1, wherein said at least one content ofthe wellbore comprises the heater cable and the characteristic isperformance of the heater cable within acceptable parameters.
 10. Themethod of claim 9, wherein the placing step comprises placing the heatercable and the distributed temperature sensor adjacent to each other inthe wellbore.
 11. The method of claim 1, wherein said at least onecontent of the wellbore comprises fluid from the formation and thecharacteristic comprises flow of the fluid from the formation into thewellbore.
 12. A system for use in a wellbore, comprising: a heater cableand a distributed temperature sensor deployed in the wellbore; theheater cable being adapted to be heated to a temperature above atemperature of at least a segment of the surrounding wellbore thereby toelevate the temperature of said at least a segment; the distributedtemperature sensor adapted to measure a temperature profile along saidleast a segment of the wellbore: and wherein the temperature profile isused to determine a characteristic of at least one content of thewellbore by using the temperature profile.
 13. The system of claim 12,wherein the heater cable and the distributed temperature sensor arepackaged in one bundle.
 14. The system of claim 12 wherein thedistributed temperature sensor comprises at least one fiber opticstring.
 15. The system of claim 12, wherein the heater cable and thedistributed temperature sensor are placed along at least one formationof the wellbore.
 16. The system of claim 15, wherein said at least onecontent of the wellbore comprises fluid from the formation and thecharacteristic is flow of the fluid from the formation into thewellbore.
 17. The system of claim 16, wherein the heater cable and thedistributed temperature sensor are placed along a plurality offormations of the wellbore simultaneously.
 18. The system of claim 17,further comprising a control unit adapted to adjust the heating level ofthe heater cable as a function of the temperature profile.
 19. Thesystem of claim 12, wherein the heating level of the heater cable isadjusted as a function of the temperature profile.
 20. The system ofclaim 12, wherein the temperature profile is used to determine whetherthe heater cable is performing within acceptable parameters.
 21. Thesystem of claim 20, wherein the heater cable and the distributedtemperature sensor are placed adjacent to each other in the wellbore.22. A method for use in a wellbore, comprising: providing a heater cableand a distributed temperature sensor; placing the heater cable and thedistributed temperature sensor in the wellbore; supplying heat to atleast a segment of the surrounding wellbore by heating the heater cableto a temperature above a temperature of said at least a segment;measuring a temperature profile along said at least a segment of thewellbore by use of the distributed temperature sensor; and determining acharacteristic of at least one content of the weilbore by using thetemperature profile.